In power semiconductor chips, a transition from the conventional Si (silicon) chips to SiC chips that excel in high-temperature operation and can increase current density of the chip has been investigated in recent years. However, where a high-density current flows in a power semiconductor chip, the calorific value of the chip increases and, therefore, the heat of the power semiconductor chip needs to be efficiently dissipated. For this reason, it is necessary to increase thermal conductivity of a heat conductive insulating sheet while ensuring excellent electric insulation ability.
In order to meet this requirement, a heat conductive insulating sheet is used which is manufactured by using a resin composition in which a boron nitride powder that has excellent thermal conductivity and electric insulation ability is contained in a matrix resin such as a thermosetting resin. Boron nitride has a layered molecular structure similar to that of graphite, and the commercially available boron nitride particles have a scaly shape. Such scaly boron nitride particles are thermally anisotropic, and thermal conductivity thereof in the plane direction (a-axis direction) of the crystal is several times to several tens of times that in the thickness direction (c-axis direction). Therefore, it is expected that thermal conductivity of a sheet in the thickness direction can be greatly increased by orienting the scaly boron nitride particles contained in the sheet such that the a-axis direction of the scaly boron nitride particles matches the thickness direction of the sheet.
Accordingly, it has been suggested to increase thermal conductivity in the thickness direction of a sheet by compounding secondary aggregated particles obtained by aggregation of scaly boron nitride particles into the sheet (see, for example, Patent Documents 1 and 2).
Patent Document 1: Japanese Patent Application Laid-open No. 2003-060134
Patent Document 2: WO 2009/04130